(Parts of the following section may not be prior art.)
Fiber lasers with high pulse energy, good beam quality and excellent optical characteristics have applications in many fields and industries such as analytical spectroscopy (fluorescence, absorption), illumination, remote sensing and environmental spectroscopy (wind speed, biohazards, ecosystem mapping etc), ranging and targeting (collision avoidance, military applications etc) and scientific instrumentation. Fiber lasers with exceptionally short pulse widths, for example, femtosecond fiber lasers, have special applications in these and other fields.
There has been great progress in developing short pulse fiber lasers. However, one of the difficulties associated with femtoseconds pulses in fibers is amplifying the pulses to high power levels. Nonlinearities in the fiber cause distortions in the spectrum, and prevent compression and propagation of high energy pulses.
A number of approaches have been developed to get around this problem. One successful technique is to amplify the ultrashort pulse in a multimode fiber. By exciting only the fundamental mode of the fiber, a large effective area can be used, thereby reducing the impact of nonlinearities.
Another possibility is to use stretched pulse amplification, where the ultrashort pulse is first stretched by many orders of magnitudes, temporally broadening the pulse and decreasing the peak power, thus eliminating or reducing nonlinear interactions. However, the pulse must ultimately be re-compressed after amplification, and the high pulse energies mean the recompression stage is usually done using bulk optics.
A fiber that is capable of propagating and compressing high energy femtoseconds pulses would then be desirable for two reasons. First, if the fiber can be designed with an appropriate dispersion, it could serve as the post compression stage for the stretched, amplified pulses. Second, if the compression function can be implemented in a fiber, it can also serve as a delivery fiber for ultrashort pulses for applications such as use in an endoscope.